A single-handle mixing valve of the type just referred to is the subject matter of U.S. Pat. No. 4,187,880 in the names of Jurgen Humpert et al. In such a valve, as well as in simpler valves with only one inlet port, it is frequently found that the displacement of the piston in surrounding outer member becomes increasingly more difficult with the passage of time. This appears to be due to diminishing lubrication as grease initially provided at the contact surfaces of the piston and the bore wall is washed out by water leakage. To remedy this problem, it has already been proposed (see U.S. Pat. No. 3,372,904 in the names of Bernard A. Graham et al) to provide the interior of the piston with a grease chamber from which lubricant can be pumped by means of a screw designed to displace a piston head in that chamber upon being rotated with the aid of an external tool such as an Allen wrench. Such a system is somewhat unhandy and calls for operation by an expert whenever relubrication is desired.